. The elasmobranch fishes. Chondrichthyes. THE ELASMOBRANCH FISHES 805 Squalus the gland is considerably increased in size and in Raja, Scyllium, and Heterodontus francisci it becomes relatively of immense dimensions. If the gland in Scyllium be taken as a type for study we find that it is divided into a dorsal and a ventral half. A section through it shows that these halves are divided into anterior and posterior areas; the former secretes albumen, the latter the shell proper. The glands which are actively engaged in secreting the shell are seen to advantage in figure 262. Here the folds are
. The elasmobranch fishes. Chondrichthyes. THE ELASMOBRANCH FISHES 805 Squalus the gland is considerably increased in size and in Raja, Scyllium, and Heterodontus francisci it becomes relatively of immense dimensions. If the gland in Scyllium be taken as a type for study we find that it is divided into a dorsal and a ventral half. A section through it shows that these halves are divided into anterior and posterior areas; the former secretes albumen, the latter the shell proper. The glands which are actively engaged in secreting the shell are seen to advantage in figure 262. Here the folds are very high and the secretory cells () at their bases are large. As the horny substance is formed for the shell it passes into the cavity of the shell gland which acts as a mold for the shell. TYPES OF EGG SHELLS Two types of shells are formed: the per- manent and the temporary shells. In the per- manent shell the young undergoes its de- velopment to the form of the adult, after w^hich it emerges {Scyllium^ Raja, Heiero- dontus). In both Scyllium and Raja the shell is a rectangle, from the angles of which pro- jections extend. These projections function either as tendrils (Scyllium.) which coil around solid objects and anchor the egg, or they serve as spikes to fix the developing egg in mud or sand flats (rays). Figure 264 shows the shell of the California swell shark, Cephaloscyllium, which in all essential re- spects is like that of the other Scyllidae ex- cept that in the figure its tendrils appear shorter. These tendrils are, however, long and are produced both from the upper and the lower angles. In color the ma- ture shell is clear amber of equal shade throughout. Such shells of Cephalo- scyllium, however, which are in the process of formation are whitish when first removed from the oviducts; but these color w^ith age. In Heterodontus the shell is shaped like a screw with a characteristic double flange extending from its apex to the large perforate end. The flange in H
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